The invention relates to a method for waterproofing the surface of a polymer workpiece.
Waterproofing makes it possible to prevent liquid or gaseous hydroxyl group (OH group)-containing substances, such as water, water vapor, or glycein, from diffusing into a polymer surface.
It is know that the diffusion of liquids, such as water or glycerin, into the polymer surface is enhanced by a specific interaction between the functional groups of the polymer surface, e.g. carbonyl groups, and the hydroxyl groups of the liquid. Consequently, various known coating processes for modifying polymer surfaces aim to neutralize the functional ester or carbonyl groups. For this purpose, the polymer surface is treated, for instance, by using an oxygen plasma so that functional groups, such as OH groups, are formed on the polymer surface. Usually, the polymer surface is then chemically coated by a grafting reaction to prevent a specific interaction of the OH groups of the liquid with the carbonyl groups of the polymer surface.
In one method of this grafting technique, the chemical substance provided for the coating is added directly as a gas to the plasma and is thereby deposited on the polymer surface. This gas phase coating process makes it possible, for instance, to form Teflon-like coatings on the polymer surface by adding C4F8, or SiO-type coatings by adding hexamethyl disiloxane. In another method of the grafting technique, the chemical substance provided for the coating is mixed into a solution for wet application and is then applied to the polymer surface by means of this solution. This method is used, for instance, to produce a silane coating on the polymer surface by treating the polymer surface with an octadecyl trimethoxy silane/toluene solution. Both of these methods, however, can be used only at locations on the polymer surface where the functional OH groups were created, for instance by the oxygen plasma treatment.
These coating methods presume corresponding ion or plasma sources and vacuum technology and thus involve substantial equipment complexity and costs. These methods are moreover time-consuming since the polymer surface to be simultaneously treated is limited to the diameter of the ion beam. A further drawback of these methods is that they are direction-dependent, i.e. good results are obtained only if the surface of the workpiece to be treated can be oriented nearly perpendicularly to the ion or plasma beam. As a result, polymer surfaces with cavities and curvatures as well as wall areas that are oriented parallel to the ion beam cannot be adequately treated.
Thus, the object of the invention is to provide a method for waterproofing the surface of a polymer workpiece, which is suitable also to treat polymer surfaces with cavities and curvatures in a simple and cost-effective manner.
The method for waterproofing the surface of a polymer workpiece is characterized by the following process steps:
a) the workpiece is treated with at least one organic solvent that swells the surface of the polymer workpiece and
b) at least one waterproofing substance, which is an organosilicon compound dissolved in the solvent, diffuses into the surface of the polymer workpiece, and
c) after a contact time, the solvent is removed from the polymer workpiece, and at least a portion of the waterproofing substance remains embedded in the surface of the polymer workpiece.
This method is used to waterproof the surface of a polymer work piece by using an organic solvent to swell the polymer surface and to serve as a carrier for at least one waterproofing substance contained in the solvent. This causes the waterproofing substance, which is an organosilicon compound, to diffuse into the surface of the polymer workpiece together with the solvent. After a contact time, the organic solvent is removed from the polymer workpiece, while at least a portion of the waterproofing substance remains embedded in the surface of the polymer work piece. This embedding advantageously takes place due to the interaction of the waterproofing substance with the polymer chains in the surface of the polymer work piece. To foster this embedding of the waterproofing substance into the surface of the polymer material, the organosilicon compound preferably has bulky organic groups, which through steric interaction with the polymer chains prevent the embedded waterproofing substance from diffusing out of the surface of the polymer material. As a result, as well as due to the hydrolysis stability of the organosilicon compound relative to water, a long-term effect of the inventive waterproofing of the surface of a polymer material is achieved. Furthermore, the treated polymer workpiece can be used in areas with elevated temperatures and/or regions with high outside temperatures.
The inventive embedding of a waterproofing substance, which is an organosilicon compound, into the surface of the polymer workpiece reliably prevents the specific interaction between the hydroxyl groups and the functional groups of the polymer matrix, e.g. carbonyl groups, and thus blocks access to the polymer matrix for liquid or gaseous hydroxyl group-containing substances, such as glycerin, water or water vapor.
The preferred treatment method of a polymer workpiece is to immerse the polymer workpiece into a bath with an organic solvent that contains the inventive waterproofing substance. After a brief contact time, during which the polymer surface swells due to the solvent and the waterproofing substance diffuses into the polymer surface, the polymer workpiece is removed from the bath. The polymer workpiece is then dried, which causes the solvent to evaporate out of the polymer matrix and at least a portion of the waterproofing material to remain embedded between the polymer chains in the polymer workpiece. Consequently, the inventive method for waterproofing the surface of a polymer workpiece can be used to treat any polymer surfaces with cavities and curvatures. This method is furthermore particularly simple and cost-effective and can be used at almost any place since only a bath and small amounts of the inventive waterproofing substance are required.
The publication by Katz et al., xe2x80x9cUltraviolet Protection of Transparent PVC Sheets by Diffusion Coatings,xe2x80x9d Proceedings of the A.C.S. Div. of Org. Coatings and Plastics, 36 (1), pp. 202-205 (1976) describes a diffusion or impregnation process of a UV absorbing material into a PVC workpiece to increase UV resistance. In this process, an organic solvent is used as the carrier for the UV stabilizer. The PVC workpiece is swelled by means of the organic solvent and the UV stabilizer is thereby introduced into the polymer workpiece. After drying, the UV stabilizer remains in the polymer surface.
Variations of this method for introducing a UV stabilizer into the polymer surface are known and are described in European Patent Application EP 0 306 006 A2. The described methods disclose no hints regarding either a method for waterproofing the surface of a polymer workpiece or the inventive waterproofing substance, which is an organosilicon compound.
Preferably the waterproofing substance is an organosiloxane, an alkyl silyl fluoride, an aryl silyl fluoride, an alkyl aryl silyl fluoride, or an alkoxy silyl fluoride.
According to a first embodiment, the organosilicon compound is an organosiloxane. Organosiloxanes comprise both linear molecules of the formula R3Sixe2x80x94[OSiR2]nxe2x80x94Oxe2x80x94SiR3 with nxe2x89xa70, as well as cyclic molecules of the formula [OSiR2]m with mxe2x89xa73, where R represents same or different organic groups. Preferably R is an alkyl group Rxe2x80x2 or an aryl group Rxe2x80x3, as specified in further detail below. An example of a linear organosiloxane according to the first formula given above is 1,1,3,3-tetraisopropyl disiloxane-1,3-diyl. An example of a cyclic organosiloxane according to the second formula given above is octamethyl cyclotetrasiloxane.
According to embodiments 2 to 5, the organosilicon compound is an alkyl silyl fluoride, an aryl silyl fluoride, an alkyl aryl silyl fluoride, or an alkoxysilyl fluoride.
Alkyl silyl fluorides according to the second embodiment comprise molecules of the general formula Rxe2x80x2nSiF4-n with nxe2x89xa72, preferably n=3, where Rxe2x80x2 represents same or different alkyl groups. The preferred groups Rxe2x80x2 are specified in greater detail below. Examples of such alkyl silyl fluorides according to the above formula with preferred groups Rxe2x80x2 are (triisopropyl)silyl fluoride, di-tert-butyl silyl difluoride and dimethyl trityl silyl fluoride.
Aryl silyl fluorides according to the third embodiment comprise molecules of the general formula Rxe2x80x3nSiF4-n, with nxe2x89xa72, preferably n=3, where Rxe2x80x3 represents same or different aryl groups. Preferred aryl groups Rxe2x80x3 are specified in greater detail below. Particularly preferred aryl silyl fluorides are diphenyl silyl difluoride and triphenyl silyl fluoride (TPSF). A particular advantage of using TPSF is its good solubility in known solvents, which are used to swell the polymer workpiece. This ensures high flexibility of the inventive method, since the surfaces of any polymer material can be treated by simply selecting a suitable solvent.
Alkyl aryl silyl fluorides according to the fourth embodiment comprise molecules of the general formula Rxe2x80x2nRxe2x80x3mSiF4-n-m with nxe2x89xa71, mxe2x89xa71 and n+mxe2x89xa72, preferably n+m=3, where Rxe2x80x3 represents same or different aryl groups and Rxe2x80x2 same or different alkyl groups. Preferred groups Rxe2x80x2 and Rxe2x80x3 are specified in greater detail below. Examples of such alkyl aryl silyl fluorides according to the above formula with preferred groups Rxe2x80x2 and Rxe2x80x3 are dimethyl phenyl silyl fluoride, diphenyl methyl silyl fluoride, tert-butyl diphenyl silyl fluoride, and (pentafluorophenyl) dimethyl silyl fluoride.
Alkoxy silyl fluorides according to the fifth embodiment comprise molecules of the general formula (RO)nRmSiF4-n-m with nxe2x89xa71, mxe2x89xa70 and n+mxe2x89xa72, preferably n+m=3, where R stands for the same or different aryl groups Rxe2x80x3 or alkyl groups Rxe2x80x2. Preferred groups are specified below. An example of a preferred alkoxysilyl fluoride is tert-butyl oxydiphenyl silyl fluoride.
It is also feasible according to the invention to use a mixture of one or more organosilicon compounds, preferably according to the above embodiments.
A preferred alkyl group Rxe2x80x2 according to the above embodiments is a linear or branched alkyl group Rxe2x80x2 with 1 to 4 C atoms, such as methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl or tert-butyl. Particularly preferably, one or more groups Rxe2x80x2 are bulky groups, such as i-propyl, i-butyl or tert-butyl. To achieve bulkiness, one or more alkyl groups Rxe2x80x2 can also have bulky substituents, such as an aryl group. An example of such a group Rxe2x80x2 is triphenylmethyl-, which is also referred to as trityl-. To increase the waterproofing action, one or more alkyl groups Rxe2x80x2 are advantageously monosubstituted or multiply substituted with fluorine.
A preferred aryl group Rxe2x80x3 according to the above embodiments is a phenyl group. To increase the waterproofing action, one or more aryl groups Rxe2x80x3 are advantageously monosubstituted or multiply substituted with fluorine.
Preferably, the waterproofing substance is used in the solvent at concentrations of between 1% by weight and 55% by weight.
In a first preferred variant of the invention, the waterproofing substance in the solvent is used at a concentration of between 1% by weight and 10% by weight. The use of such small amounts of the waterproofing substance, e.g. triphenyl silyl fluoride, is preferred particularly for the waterproofing of optical polymer surfaces, since this makes it possible to retain the optical quality of the surfaces, e.g. their reflection properties.
In a second preferred variant of the invention, the waterproofing substance in the solvent is used at a concentration of between 10% by weight and 55% by weight. With the use of these correspondingly larger percentages of the waterproofing substance in the solvent, a layer with a particularly high content of the waterproofing substance forms in the polymer surface. Furthermore, our own tests have shown that the use of the correspondingly large percentages of the waterproofing substance in the solvent enables the formation of a layer of the waterproofing substance on the polymer surface. These tests have also shown that the formation of this layer on the polymer surface can be prevented if the polymer workpiece is rinsed with a solvent, e.g. ethanol, isopropanol or toluene, immediately after the inventive treatment.
Preferably, the workpiece is treated at a temperature of below the melting temperature of the waterproofing substance. The upper temperature limit of the dipping bath must be selected below the glass transition temperature of the polymer workpiece to be treated to ensure the dimensional stability of the workpiece. The lower temperature limit of the dipping bath is determined, respectively, by the polymer workpiece to be treated and by the selection of the solvent based on the swellability of the polymer workpiece.
Preferably, the workpiece is treated at a temperature ranging between 0xc2x0 C. and 60xc2x0 C. This makes it possible to use simple heating devices to heat the dipping bath. Since it is especially preferred to carry out the treatment of the workpiece in the dipping bath at room temperature, i.e. between 10xc2x0 C. and 30xc2x0 C., it is even possible to dispense with a corresponding heating device.
Preferably a mixture of at least two organic solvents is used. At least the first solvent should be capable of swelling the surface of the polymer workpiece and at least the second solvent of dissolving the hydrophobic substance. This results in a wide range of possible selections for both the first and the second solvent. The two solvents and their corresponding percentages by weight are selected as a function of the polymer workpiece to be treated and the waterproofing substance. For instance, acetone is preferably used in small amounts to dissolve the waterproofing agent, such as triphenyl silyl fluoride.
For the organic solvent, one or more solvents is preferably selected from the group comprising the low-molecular (C1-C10) saturated or unsaturated, linear, branched or cyclic, possibly substituted alkanes, alcohols, ethers, esters, aldehydes, ketones, N,N-dialkyl amides, aromatic compounds. Examples of the solvents of the above group are hexane, heptane, octane, nonane, decane, decahydro naphthalene, methanol, ethanol, propanol, hexafluoropropanol, butanol, pentanol, hexanol, di-n-butyl ether, tert-butyl methyl ether, butyl acetate, tetrahydrofuran, methyl-, ethyl-, propyl-, butyl- or pentylacetate, acetone, hexafluoroacetone hydrates, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, cyclohexanone, N,N-dimethylformamide, N,N-dimethyl acetamide, toluene, or xylene. To swell the surface of the polymer workpiece, preferably a polar solvent is used for polar polymers and preferably a solvent of low polarity for non-polar polymers.
Particularly preferred organic solvents are butyl acetate, acetone and/or toluene. In the first variant of the invention, the use of butyl acetate is preferred since this solvent readily dissolves the TPSF active substance and also ensures that the optical quality, e.g. reflection properties, of the surfaces is retained when, for instance, PMMA polymer surfaces are treated. The use of toluene is preferred for short residence times of the polymer workpiece in the solvent.
Preferably, the polymer workpiece comprises a thermoplastic or an elastomer polymer material. Examples of such polymer materials made of a thermoplastic material are polystyrene (PS), polypropylene (PP), polyethylene (PE), cyclo-olefine copolymer (COC), polymethylmethacrylate (PMMA), polycarbonate (PC), polyoxymethylene (POM), polysulfone (PSU), polyphenylene ether (PPE), polyetheretherketones (PEEK), polyether imide (PEI), polybutylene terephthalate (PBT), polyacrylate, self-reinforcing partially crystalline polymers (LCP), polyethylene terephthalate (PET), polyvinylidene fluoride (PVDF), cyclo-olefin polymer (COP), polyvinyl acetate, polyvinylidene chloride, a copolymer based on acrylonitrile, butadiene and styrene (ABS) or a copolymer based on acrylates and ethylene. Examples of such polymer materials made of an elastomer are polyurethane (PUR), polybutadiene (BR), ethylene propylene terpolymer (EPDM), nitrite rubbers (NBR), styrene butadiene rubber (SBR), and natural rubber (NR).
Preferably, the polymer workpiece is treated with a contact time of less than 2 hours, preferably less than xc2xd hour. Thus the inventive waterproofing of the surface of a polymer workpiece requires very little time.
Preferably, the waterproofing substance remains in the surface of the polymer workpiece to a penetration depth of less than 50 xcexcm, particularly less than 20 xcexcm. This shallow penetration depth of the waterproofing substance, e.g. TPSF active substance, into the surface of the polymer workpiece prevents mechanical deformation and stresses in the polymer workpiece and thus the formation of cracks along the polymer surface.
Preferably, the solvent is removed by applying a vacuum to the polymer workpiece and/or by heating the polymer workpiece.
Removal of the solvent by applying a vacuum to the polymer workpiece is particularly preferred in polymer workpieces with cavities, since this requires little time to remove the solvent from the cavities.
In polymer workpieces with curvatures, smooth surfaces and/or cavities that are directly accessible from the surface, the solvent is preferably removed from the polymer workpiece by heating or drying. The preferred drying temperature ranges from 10xc2x0 C. to 60xc2x0 C. This prevents the waterproofing substance from diffusing out of the polymer surface. It also makes it possible to use simple and inexpensive drying equipment. Especially preferred is the use of room temperature (approximately 10xc2x0 C. to 30xc2x0 C.) for drying, since this eliminates the need for drying equipment. In the case of the first preferred variant of the invention and the use of low percentages of the waterproofing substance, the treated polymer workpiece is preferably dried at room temperature. In general, the polymer workpiece can be used for its intended purpose directly after removal of the solvent from the polymer surface.
A particular advantage of the invention is that the inventive waterproofing makes it possible to treat surfaces of a polymer workpiece and workpieces with polymer surfaces of any shape and size. This makes it possible to prevent diffusion of liquid or gaseous hydroxyl group-containing substances, e.g. water, water vapor or glycerin, including, e.g., in any component housings made of a polymer material of optical, mechanical, electronic or other components, with little time being required and at low costs.
Additional aims, advantages, features and possible applications of the present invention result from the following description of several exemplary embodiments with reference to the drawings. All the described and/or depicted features are the subject of the invention either per se or in any meaningful combination, irrespective of their summarization in the claims or the referencing.